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Before tackling complex scenarios, it is essential to master the foundational tools for active troubleshooting. The Looking Glass utility and the ARP table provide definitive, real-time answers to fundamental questions about connectivity and reachability.

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Looking Glass

The Looking Glass tool is a suite of active diagnostic utilities that allow you to test the network path from the perspective of the platform itself.
ToolFunctionPrimary Use Case
Route LookupVerifies route visibility from a location.”Is my new VPC route correctly advertised to my branch office?”
PingTests end-to-end reachability and latency.”Can my server reach the public internet?”
TracerouteTraces the hop-by-hop path to a destination.”Where is the latency or packet loss occurring on the path to my application?”
Path MTU DiscoveryDetermines the maximum packet size on a path.”Is a mismatched MTU causing performance degradation?”
Packet CaptureCaptures raw network traffic for deep analysis.”I need to analyze the full packet flow to debug a complex protocol issue.”

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ARP Table

The Address Resolution Protocol (ARP) Table is another fundamental tool, used to validate Layer 2 connectivity and provide crucial clues for Layer 3 troubleshooting. By navigating to Network -> Cloud Router -> ARPs, you can see the mapping of IP addresses to MAC addresses on a given interface.

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Use Case: Troubleshooting BGP

The presence of an ARP entry for a BGP peer’s IP address is a definitive confirmation that Layer 2 connectivity (e.g., the correct VLAN, physical link) is established. If BGP peering is failing, but a valid ARP entry exists for the peer, the problem is almost certainly a Layer 3 BGP configuration error (such as an incorrect AS number or a mismatched authentication key) and not a physical or VLAN issue. This allows you to immediately focus your efforts on the correct configuration domain.